This invention relates to servo positioning the read heads of a video cassette recorder (VCR), and particularly to detecting pilot-tones from adjacent tracks or stripes and digitally processing pilot-tone information for centering the head on the track being followed.
Video cassette recorders record audio and video signals in stripes across the width of the tape, the stripes being arranged in parallel at an incline to the direction of the tape travel. As illustrated in FIG. 1, the stripes are recorded such that every other stripe along the length of the tape contains information modulated to produce a tone at either of two pilot-tone frequencies, f1 and f2. The f0 stripes are interleaved between the f1 and f2 stripes such that each f0 stripe is bounded by adjacent f1 and f2 stripes. Each f0 stripe contains specific notches at the f1 and f2 frequencies. Track following occurs only when the head is attempting to track the f0 stripes. As shown in FIG. 1, the head senses the pilot-tones of the adjacent f1 and f2 stripes. The amplitude of each pilot-tone f1 or f2 sensed is largely affected by the position of the head in relation to the f1 and f2 stripes. If the head detects an f1 pilot-tone amplitude greater than an f2 pilot-tone amplitude, the head is off the center of the f0 stripe toward the f1 stripe. Differential averaging techniques are employed to quantify the off-center condition of the head and to operate a servo mechanism based on the f1 and f2 pilot-tone amplitudes to move the head relative to the f0 stripe until the head is centered on the f0 stripe and the f1 and f2 pilot-tone amplitudes are equal.
Prior pilot-tone detection and processing employed analog technology using bandpass filters that pass signal amplitudes centered on the pilot-tone frequencies. The amplitude of each filtered signal was detected and the difference between them was a measure of servo mis-position. The difference signal was ultimately converted to a digital form to operate the servo control microprocessor. However, pilot-tone frequency can vary by as much as xc2x110% due to variations in tape speed, such as in a xe2x80x9ctrickxe2x80x9d play mode. Therefore, bandpass filters associated with pilot-tone detection necessarily had to be wide enough to accommodate expected frequency variations of the pilot-tones. Wider bandpass filters resulted in a greater likelihood of detection of noise with the pilot tone, resulting in inaccurate amplitude detection of the pilot-tone. Inaccuracies in pilot tone amplitude detection resulted in errors in operating the servo to center the head on the f0 stripe. Moreover, certain servo algorithms cannot process wide bandwidths, making the wide bandpass filters infeasible in some systems. Narrow bandpass filters could be employed but required switching to define the bandwidths of the actual tones, thereby adding to the complexity of the detection and processing of pilot-tones. The present invention is directed to a digital technique for digitally processing the pilot tone frequencies to inaccuracy of less than about 0.5%, over a range of xc2x110% of the nominal pilot-tone frequency. As a result, pilot-tone amplitude is more accurately detected, resulting in greater accuracy of the head positioning over the f0 stripes, and the quantity of expensive analog circuitry can be reduced.
According to the present invention, the amplitudes of at least two signals having mutually exclusive frequencies are digitally determined from an input digital representation of an input signal set containing the two signals. Digital representations are locally derived representing the nominal frequencies of the two signals, from which digital representations of the sine and cosine functions of each locally derived frequency are derived. The derived digital representations of the sine and cosine functions of each frequency are mixed with the input digital representation to determine a product of the input signal with the sine and cosine functions of each of the locally derived frequencies. The resulting four digital representations are processed to derive digital representations of the amplitudes of each of the two signals.
In one form of the invention, the digital representations of the frequencies are derived by establishing a number representative of the respective frequency and repeatedly advancing a count in an accumulator by the number to establish a digital ramp having a slope representative of the frequency. One accumulator is provided for each frequency f1 and f2. The digital representations of the sine and cosine functions of each of the two locally derived frequencies are stored in a look-up table or an array of look-up tables, and the sine or cosine value is based on an instantaneous value of the count in the accumulator. The sine and cosine functions of each frequency are multiplied with the input signal, and the output of each multiplier is processed in a separate digital low-pass filter to extract the baseband component thereof. The outputs of the low-pass filters processing the sine and cosine functions of frequency f1 are combined in each channel in root-sum-of-squares fashion to yield a signal representative of the amplitude of the f1 component of the input signal. In similar fashion, the outputs of the low-pass filters processing the sine and cosine functions of frequency f2 are combined in root-sum-of-squares to yield a signal representative of the amplitude of the f2 component of the input signal. The signals representative of the amplitudes of f1 and f2 are subtracted and the result is provided in digital form to the servo control microprocessor of the VCR that regulates head position.
Another form of the invention employs time-sharing techniques to derive the digital representations of the sine and cosine functions and to perform the mixing, low-pass filtering and root-sum-of-squares combining operations, thereby achieving significant savings in the hardware volume utilized to implement the invention. The amplitudes of at least two signals having mutually exclusive frequencies are digitally determined from an input digital representation of an input signal set containing the two signals. Digital representations are derived representing the frequencies of the two signals at frequencies f1 and f2. As in the first form of the invention, these representations are obtained by incrementing an accumulator at an appropriate rate for each frequency, and two accumulators are therefore provided. A sine/cosine look-up table, mixer, low-pass filter and root-sum-of-squares combinor are employed sequentially and in pipeline fashion to process the four channels of information representing sine and cosine functions of signals for both frequencies f1 and f2. State memory is employed to store the states associated with each channel.